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I'm new to Python, and I want to test if a certain point lies ON the (boundary) of a polygon - not inside, not outside, just if it's on the boundary. The points and polygon already defined 

polygon = [(3, 2), (5, 1), (7, 2), (8, 6), (9, 7), (11, 6), (10, 4), (12, 2), (15, 2), (16, 5), (19, 7), (18, 11), (14, 12), (11, 9), (5, 9), (2, 6)] 
Point_X = 14
Point_Y = 12

this point should be a boundary. How can I do that?

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  • 1
    Welcome to GIS SE. Thank you for taking the Tour. Due to the imprecision associated with floating-point values, far fewer points which are "on" a line will actually be reported as such in real-world situations. If you want to test this, your code needs to use ArcPy functions to assemble Polygon and PointGeometry objects, so that you can cast the Polygon to a line (using Geometry.boundary()) and test for point intersection with Geometry.contains(). Be sure to specify an appropriate SpatialReference. – Vince Jan 14 '18 at 19:37
  • You had a tag for ArcPy but your question body and title make no mention that it is being used so I am removing it. – PolyGeo♦ Jan 14 '18 at 20:22
  • I'm also adding a tag for shapely to match the answer. If you are not using that library then please ask a new and more focused question. – PolyGeo♦ Jan 14 '18 at 21:18
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    @PolyGeo You've altered the question twice to make it fit an answer that did not help the OP, and in so doing, made my comment nonsensical. This was not a greatest moment in moderation. – Vince Jan 15 '18 at 12:25
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    @PolyGeo Saw that. Busy day here today, and probably tomorrow. I'll see if I can do better while caged at home after the snow storm. – Vince Jan 16 '18 at 5:19
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Here's an example using shapely, based on the shapely documentation. https://shapely.readthedocs.io/en/stable/manual.html#binary-predicates

from shapely import geometry

polygon = [(3, 2), (5, 1), (7, 2), (8, 6), (9, 7), (11, 6), (10, 4), (12, 2), (15, 2), (16, 5), (19, 7), (18, 11), (14, 12), (11, 9), (5, 9), (2, 6)] 
Point_X = 14
Point_Y = 12

line = geometry.LineString(polygon)
point = geometry.Point(Point_X, Point_Y)

print(line.contains(point))
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  • The original question was unclear, but it was tagged "arcpy", which was an indication that this answer would not be useful to the OP. Please hold off on answering unclear questions, since it is not usual that a question is altered to make the inappropriate answer correct. – Vince Jan 15 '18 at 12:17
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    Doc URL mentioned above is now at: shapely.readthedocs.io/en/stable/… – J Graham Oct 28 '20 at 14:49

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